1. Technical Field of the Invention
This invention relates to a two-way communications device that uses the vibrations of an eardrum membrane caused by a user's voice, and more particularly, to a two-way communications device that uses one transducer for both receiving and sending voice. The communication device also serves as a high-level echo canceller (EC) and as a high-level voice-operated exchanger (VOX).
2. Description of the Related Art
A generally known device used for two-way voice communication includes a microphone and an earphone. The microphone and earphone are integrated as a headset providing “hands free” operation for the user. One problem with these headsets is that the microphone also picks up ambient noises around the user.
As an alternative, a user's voice is picked up not at the mouth, but through bone conduction within the user's ear canal. This provides “mouth-free” operation for the user (i.e., the user can keep the periphery of his or her mouth free). Voice signals received through bone conduction suppress peripheral noises to some degree, however, the sound quality of voice signals detected via bone conduction generally have bad sound quality.
Some headsets have been proposed that use two transducers. A first transducer is used as a microphone and a second transducer is used as an earphone. The two transducers are inserted into a user's right and left ears. The microphone transducer detects the user's voice via vibrations from the membrane in the user's eardrums. In this two transducer system, ambient noises around the user are naturally suppressed allowing the microphone to provide better sound quality.
The use of single transducer headsets has also been advanced. The idea is to use a single transducer element for both voice transmission and reception. With this method, only one earplug is needed. Since the user's other ear does not require an earphone, the user can also hear sounds and voices around him or her.
It is important for these devices to have an echo-cancellation (EC) function. The echo cancellation prevents reception signals from being superimposed on voice transmission signals. In some cases, the headset also includes a voice-operated exchanger (VOX). The VOX switches between voice transmission and reception modes according to the presence or absence of transmission and/or reception signals.
Japanese Patent Application Publication No. 2001-60895 discloses a transmission-and-reception circuit that utilizes an all-analog circuit that is equipped with echo-cancellation and a voice-operated exchanger. A bridge circuit in the disclosure combines an amplifier and comparator with an embedded single transducer.
However, it is difficult and expensive to achieve reliable and satisfactory EC and VOX performance with these types of analog circuits. Because of these difficulties, the above-mentioned single transducer transmission-and-reception circuits have not yet come into practical use.
When designing an EC circuit, it is necessary to simulate the actual impedance characteristics of the transducer. Namely the characteristics of the transducer when inserted into a particular user's external ear canal must be simulated, in order to balance the EC circuit with the actual transducer. However, it is difficult to simulate this impedance due to the inductive properties of the transducer. Furthermore, the actual impedance of the transducer varies over time and also varies according to the individual user and according the surrounding environment.
An ordinary analog-simulation circuit consists of capacitors (Cs) and resistors (Rs). The analog circuit can be approximately balanced at a single specific frequency by tuning two variable elements. It is impossible to achieve balance over an entire frequency range. In theory it might be possible to achieve balance over the entire frequency range, if an inductance element is included in an analog-simulation circuit. However, using an inductance element is bulky and expensive. It is also very difficult to tune an inductance element to conform with the varying transducer characteristics. This makes using analog inductor circuits impractical for headset applications.
In the case of half-duplex two-way communications, a VOX is essential. The VOX monitors reception and transmission voice signals, then must determine in less than a few milliseconds whether to select a transmission mode or a reception mode.
During a conversation, the received and transmitted voice signals continually vary in magnitude and some times are also intermittently disconnected. Therefore, it is necessary continuously accumulate and process voice data up to the time when a determination is made whether to switch to the reception mode or the transmission mode. This determination process is difficult, especially when using an analog-circuit.
Embodiments of the invention address these and other disadvantages of the conventional art.